A plurality of demands which are often contradictory are made on chippers. The most important demand is that the chipper must be able to produce wood chips of a quality that is as high and uniform as possible. Among other things the produced wood chips have to be of uniform size and the amount of oversized chips, as well as the amount of fine material in the form of splinters and shavings have to be small in order to obtain a high yield of the raw material of wood.
Another demand or object to be desired is that the total operative expenses should be as low as possible. For instance, the costs for replacing and/or regrinding of the chipper knives must be low.
An important factor as regards the production of chips of a high and uniform quality is that the top surface or the flank of the chipper knife, i.e. the outermost surface of the chipper knife which during the cutting faces the logs being fed to the chipper, has such a shape and size that during the cutting it interferes with the feeding of the wood as little as possible. In drum chippers the ideal shape of the flank is slightly arched in cross-section with a radius of curvature that corresponds to the radius of curvature of the drum. In disc chippers the ideal shape of the flank is helicoidal, i.e. has a propeller-like shape with a greater angle to the plane of rotation at the centre of the disc than at the periphery of the disc.
Producing chipper knives with such a complicated shape of the flank is technically possible but, in practice, it usually becomes too expensive. On the market there are, in fact, chippers of a large number of makes, types and sizes and, in order to limit the costs, one and the same knife system has to be generally useful in a large number of different chippers without needing to be specially worked and kept in stock for each type of chipper. As a rule, it is thus preferred to produce chipper knives with a constant edge angle and a plane flank and only to twist or arch the other surface parts of the disc and the drum, respectively, which contact the wood. For such a chipper knife to interfere with the feed of the wood as little as possible, it is advantageous if the flank is made with a cross-sectional length that is as small as possible, so that the wood as early as possible should pass over and be guided towards the surfaces of the chipper that follow the chipper knife and that have a correctly arched shape or twisted shape. Such a design makes a good compromise between the demands for a high chip quality and low costs. However, the physical properties of the chipper knife often set lower limits as regards how small the flank can be made since the chipper knife has to have a predetermined minimum cross-sectional thickness in order to resist the strain to which it is exposed.
An important factor when it comes to keeping down the operative expenses is to reduce the time expenditure when adjusting the chipper knives, i.e. the time required for positioning the chipper knives in the chipper when dismantling worn chipper knives and mounting new ones. Most advantageously, the need of adjusting the chipper knives is completely eliminated by the chipper knives not needing to be positioned, but having a predetermined and well-defined position in the holder of the chipper. Moreover, a fixed positioning of the chipper knives has the advantage of these not risking sliding out of the holders or being turned in connection with unbalanced load when in operation. However, in such a knife system, it is not feasible to regrind worn chipper knives without time-consuming handling since this implies that their position in the holder is changed after regrinding, but instead the worn chipper knives are discarded. By forming such chipper knives as symmetric indexable inserts with two opposite cutting edges which, by turning of the chipper knife, can be located in a chipping position, their service life can certainly be doubled, but the cost of production of the chipper knife is after all important as regards the total operative expenses. An important factor as regards keeping down the operative expenses will thus be to be able to limit the cost of production of such chipper knives and, above all, to limit the amount of material in the chipper knife, i.e. make it as small and thin as possible and thereby reduce the material costs. However, this cannot be easily achieved since the chipper knife for reasons of function and strength of material needs to have a certain size and, especially, a minimum thickness in certain cross-sections subjected to heavy loads.